Method and apparatus for strengthening/restoring a reinforced/prestressed concrete structure

ABSTRACT

A reinforced or prestressed concrete structure is strengthened and/or restored by attaching a strip-shaped tensional member thereto. Prior to attaching the tensional member, a central portion thereof is prestressed. Then, the tensioned central portion is bonded to a concrete surface of the concrete structure by a first adhesive having a high deformation modulus. Thereafter, the non-prestressed end portions of the tensional member are bonded to the concrete surface by a second adhesive having a low deformation modulus, whereby the second adhesive is substantially less deformable than the first adhesive.

BACKGROUND OF THE INVENTION

The invention related to a method and strip-shaped tensional member forstrengthening and/or restoring reinforced or prestressed concretesupporting structures.

It is well known to attach prestressed tensional members to the outsideof supporting structures to increase the load carrying capacity(strengthening) or to restore to the original load carrying capacity(restoration) supporting structures made of reinforced or prestressedconcrete.

For example, brackets made of steel or reinforced concrete are anchoredwith dowels to the concrete surface. However, due to a relative shifting(displacement) between the axis of the tensional member and the concretesurface there is an unfavorable shift movement created, which has to beabsorbed and transferred by the bracket anchoring; in addition, a bondbetween the tensional member and the concrete surface cannot beproduced.

To avoid disadvantages caused by the shift between the tensional memberand the concrete surface, it has been disclosed in a method of the typementioned in the beginning to later attach, with an adhesive,strip-shaped tension members for strengthening and/or restoringreinforced or prestressed concrete support structures. Such a continuousadhesive joint is especially of significance if cracks are present orcould occur in the concrete or further widening of the cracks is to beprevented.

If tensional members are bonded to a concrete surface in a stressedcondition, then the adhesive joint becomes stressed over a period oftime by the introduction of the shearing (transverse) force as a resultof the prestress force. Adhesive agents with the lowest possibledeformation modulus are used, which offer a rigid adhesive joint to thegreatest extent in its hardened condition to avoid creep losses as aresult of this shearing stress on the adhesive joint.

High-strength (high tensile strength), thin, and thereby comparablylight strips are preferably employed as tensional members for easyhandling. The strips made of high-strength material are usually linearelastic up to the point of fracture; an upper flow level is not presentin such high-strength materials, which means an area of near constantforce over the path of expansion.

If thusly constructed band-shaped prestressed tensional members covercracks in the concrete surface, which open up during added stress on thesupport structure, then there is the danger with the described rigidadhesive Joint that the tensional member is stretched passed the crackedges in the crack area to such an extent that there occurs a suddenfracture in this type of strip without prior indication of elasticdeformation.

This lack in ductility is a disadvantage and considerably limits thepossible utilization factor of the tension members. Therefore adhesivejoint agents must be selected for use in the areas in which cracks areto be covered, which have a specific deformation characteristicthemselves, which means a high deformation modulus, so that duringopening of the cracks an expansion compensation can take place in thetensional member over a length that is clearly greater than the degreeof added widening of the cracks. However, such types of adhesive jointagents with high de formation modulus are not suited for permanentanchoring because of the associated creep losses.

In addition, there is the problem that at both ends of the prestressedtensional members the tie force has to be introduced into the minorstructure (substructure). In traditional prestressed concrete structuresthere is provided a so-called rear suspended reinforcement therefor,which prevents development of cracks immediately behind the anchoring ofthe tensional member. Such a rear suspended reinforcement is alsonecessary in later attached prestressed tensional members.

In a known method of the type mentioned in the beginning (WO 97/21009,FIG. 5) there is a tensional member provided with fiber components,which is glued—while prestressed at its middle area of its length—to theconcrete surface of a supporting structure. The tensional member isclamped at its two end areas into a clamping device, which is joined tothe supporting structure via a ductile deformable transition element.The ductile deformable transition element is a bonding agent, whichdeformation modulus is substantially higher than that of the adhesivejoint agent in the middle area of the tensional member. Since theanchoring of the tensional means at its end areas is effective onlyduring excessive load conditions on the tensional means over the entiremiddle area, which means only in case of local excessive loads, thedanger of fracture in the area of cracks is thereby not a voided.

The object of the invention is to overcome the cited disadvantages andto provide a method for strengthening and/or restoring reinforced orprestressed concrete supporting structures, whereby the danger of afracture in the areas of cracks is avoided through the avoidance of ashift moment during the direct attachment of the strip-shaped tensionalmember on the concrete surface, and whereby creep losses are excluded.

SUMMARY OF THE INVENTION

This object is achieved according to the invention in that the seconddeformation modulus is substantially lower compared to the firstdeformation modulus.

In the prestressed area, which extends over the most part of the lengthof the tensional member, there axe from the start no shearing forcestransmitted via the adhesive joint agent between the tensional memberand the concrete surface. Only when expansions and particularly cracksdevelop in the concrete, then there occurs a transfer of shearing forcesin local, small areas between the tensional member and the concretesurface. However, since there is an adhesive joint agent used withcomparably soft shearing properties in the middle area of the tensionalmember, there occurs an expansion compensation in the tensional memberover the width of the crack so that, in general, a ductile process isemployed.

The extensions at the two end areas of the tensional member, which axenot prestressed, serve as anchoring and rear suspended reinforcement.Since an adhesive joint agent with low deformation modulus—which means agenerally rigid adhesive joint agent—is used in the area that is notprestressed, the prestress force is transmitted into the concretewithout creep losses to a great extent.

The surface-bonding arrangement of the strip-shaped member prevents thedevelopment of a shift moment. A separate anchoring and rear suspendedreinforcement is not necessary since these functions are achieved in anespecially simple and space-saving fashion by the rigid adhesiveconnection of the non-stressed end areas of the tensional member withthe concrete surface.

The invention relates further to a strip-shaped tensional member forstrengthening and/or restoring concrete supporting structures. Startingwith a strip-shaped tensional member for strengthening and/or restoringconcrete supporting structures that is prestressed in a middle area ofits length and which is joined to said concrete surface by means of afirst bonding agent with a first deformation modulus, and which joinedat its two end areas, which are not in a prestressed condition, by meansof a second bonding agent with a second deformation modulus, the noveltensional member is characterized in that the second deformation modulusis substantially lower in comparison to the first deformation modulus.The tensional member may consist of steel fibers, synthetic fibers, orpreferably carbon fibers.

Finally, the invention also relates also to a device to carry out themethod (process) of a strip-shaped tensional member being arranged on aconcrete surface with tensioning devices and end anchoring. This device,according to the invention, is characterized in that the tensioningdevice is provided with a force introduction body that is joined bypositive fit to strip-shaped tensional means, which may be moved bysliding in a longitudinal direction of the tensional means relative to abase body attached to the supporting structure, wherein a tensioningdrive may be inserted between the force introduction body and the basebody, and wherein at least one spacer may be inserted between thebuttress surfaces of the base body, which face one another, and theforce introduction body.

This tensioning device, which is preferably disposed at one orpreferably at both ends of the middle prestressed area of the tensionalmember, is able to supply the necessary prestress force on the middlearea of the tensional member in a structural and space-saving fashionwhereby the two ends of the tensional member, which are outside theforce introduction points, are not prestressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, an embodiment example of the invention is described inmore detail with reference to drawings.

FIG. 1 shows in a simplified illustration a reinforced or prestressedconcrete supporting structure with a tensional member attached thereon.

FIG. 2 shows a tensioning device in its initial condition disposed atone force introduction point taken at a view in the direction of thearrow II in FIG. 1.

FIG. 3 shows the tensioning device according to FIG. 2 after thetensioning process.

FIG. 4 depicts a view of the tensioning device taken in the direction ofthe arrow IV in FIG. 2.

FIG. 5 depicts a view of the tensioning device taken in the direction ofthe arrow V in FIG. 3.

FIG. 6 illustrates the tensioning device in an exploded view accordingto FIG. 2 through FIG. 5.

FIG. 7 shows in an illustration according to FIG. 1 a reinforced orprestressed concrete supporting structure comprising a tensional memberthat is attached in a modified process.

FIG. 8 shows in a schematic longitudinal view the ends of a plurality oftensional members lying on top of one another.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

For strengthening or restoring a reinforced or prestressed concretesupporting structure 1 there is a strip-shaped tensional member 3attached to its concrete surface 2 whereby said tensional memberconsists of carbon fibers, for example. The middle area 3 a of thetensional member 3 is prestressed between two force introduction points4 by a prestress force indicated by arrows 5 in FIG. 1. The tensionalmember 3 is not prestressed at its two end areas 3 b that are disposedoutside said force introduction points 4.

In its middle prestressed area 3 a, the strip-shaped tensional member 3is joined to the concrete surface 2 by means of a first adhesive jointagent, which has a relatively high deformation modulus. At its two endareas 3 b, said tensional member 3 is joined to the concrete surface 7by means of a second adhesive joint agent, which has, in contrast, asubstantially lower deformation modulus; this second adhesive jointagent is substantially rigid in its hardened condition.

If during stress conditions on the supporting structure 1 an existing ordeveloping crack 6 opens up in the concrete surface 3, then an expansioncompensation occurs in the tensional member 3 on both sides of the crack6 as a result of the deformation characteristics of the adhesive jointagent.

A tensioning device 7 is arranged at both force introduction points 4 toapply a prestress force to the middle area 3 a of the tensional member3, as illustrated in FIG. 2 through FIG. 6.

Each tensioning device 7 is provided with a base body 8 in the shape ofa plate, which is attached to the bottom of a flat recess 9 in theconcrete surface 2, preferably by adhesion.

A force introduction body 10, which is T-shaped in its horizontalprojection, is attached to the under-side of the strip-shaped tensionalmember 3 along the T-extension 10 a whereby it is preferably adheredthereto and whereby it is disposed in a recess 11 of the base body 8 insuch a manner that the force introduction body 10 is also countersunk inthe recess 9 of the concrete surface 2. The joint surface that is joinedpreferably by adhesion to the tensional member 3 (which means the topsurface of the T-extension 10 a of the force introduction body 10) liesin the plane of the concrete surface 2.

The connection between the T-shaped force introduction body 10 and thestrip-shaped tensional member 3 may be alternatively designed asfollows:

1. The T-extension 10 a may be designed to be longer that illustrated.

2. A second T-shaped force introduction body (not shown) may be arrangedon the opposite side of the first force introduction body 10 and may bejoined to the tensional member 3 and the force introduction body 10.

3. Fins, for example steel fins, may be adhered to the tensional member3 and attached to the force introduction body 10—also by adhesion or bypositive fit—to improve force introduction into the strip-shapedtensional member.

At the two T-cross sections 10 b of the force introduction body 10 thereengages a hydraulic or mechanical tensioning element of a tensioningdrive 12, which is braced against the base body 8. In the illustratedembodiment example, the tensioning drive 12 is provided with twohydraulic tensioning cylinders 12 a disposed on both sides of theT-extension 10 a, which engage at both T-cross sections 10 b. An angledcovering 13 is connected to the base body 8 and serves as an attachmentfor the tensioning drive 12.

The tensional member 3 is prestressed between the tensioning drives 7 ofthe two force introduction points 4 after application of its middle area3 a to the concrete surface 2 and before hardening of the first adhesivejoint agent. At least one spacer 14 is inserted between buttresssurfaces 10 c of the force introduction body 10 and the buttress surface8 a of the base body 8 that face one another to mechanically attach thetensional member with a positive fit (relative to the concrete surface)to at least one, but preferably to the two force introduction points 4after employing the prestress force. In the illustrated embodimentexample, the spacer 14 consists of several pieces of sheet metal withwhich the force introduction body 10 wedged in place opposite to thebase body 8.

The tensioning drive 12 may be removed from the tensioning device 7after the wedging process so it can be employed to prestress anothertensional member.

The force introduction body 10 is preferably also provided with anadhesive joint agent on its under-side, which substantially hardens intoa rigid adhesive joint after the tensioning process.

The end areas 3 b of the tensional member 3 are attached to the concretesurface 2 by means of an adhesive joint agent, which is substantiallyrigid after hardening. The end areas 3 b that are attached in thisfashion to the concrete surface 2 form a rear suspended reinforcementfor the two ends of the tensional member 3.

In the presently described embodiment example, according to FIG. 1, theprestress force for the middle area 3 a has been introduced at two forceintroduction points 4 that are disposed between the middle area 3 a andthe adjoining end area 3 b, respectively. As a modification thereofthere is illustrated in FIG. 7 that the prestress force 5 may beintroduced via the force introduction points 4′ that are disposed at theends of the tensional member 3 after the first adhesive joint agent hasbeen applied to the middle area 3 a. After this first adhesive jointagent has hardened, then the ends of the tensional member 3 areseparated (by cutting, for example) from the tensioning devices that aredisposed at the force introduction points 4′. The end areas 3 b of thetensional member 3 are folded upright and coated with a second adhesivejoint agent and then they are attached to the concrete surface with saidadhesive joint agent.

In FIG. 8 it is shown that several strip-shaped tensional members 3, 3′may be applied on top of each other, one after the other. At first, thelowest strip-shaped tensional member 3 is attached to the concretesurface 2 in the already described manner. After hardening of theadhesive joint agent under the middle area 3 a and the end area 3 b, asecond strip-shaped tensional member 3′ is laid on top of the firststrip-shaped tensional member again with an adhesive joint agent thathas a high deformation modulus (illustrated by a wavy line.) Thetensioning device 7 that is attached to both ends provides the necessaryprestress force.

After the adhesive joint agent has hardened under the middle area 3 a′,then the end of the tensional member 3′ is separated from the tensioningdevice 7 in the manner described above. The end areas 3 b′ are foldedupright, coated with an adhesive joint agent with low deformationmodulus, and then glued to the concrete surface 2.

This process may be repeated several times by applying one after theother a plurality of tensional members on top of each other whereby therespective end areas 3 b, 3 b′ . . . are attached rigidly, directly ontothe concrete surface 2.

The tensioning device 7 may be completely removed from the concretesurface 2 after all tensional members 3, 3′ have been applied; only thebase plate 6 may remain, whereas all other components of the tensioningdevice 7 are removed.

What is claimed is:
 1. A method for strengthening/restoring areinforced/prestressed concrete structure, comprising the steps of: A)prestressing a middle portion of a length of a strip-shaped tensionalmember; B) joining the prestressed middle portion to a concrete surfaceof the concrete structure by a first bonding agent having a firstdeformation modulus; and C) joining two non-prestressed end portions ofthe tensional member to the concrete surface by a second bonding agenthaving a second deformation modulus substantially lower than the firstdeformation modulus.
 2. The method according to claim 1 wherein steps Band C comprise using an adhesive as the first and second bonding agents.3. The method according to claim 2 wherein step A comprises engaging themiddle portion against the concrete surface, with the first bondingagent disposed therebetween, and then pre-stressing the middle portionby applying tension to two points on the tensional member before thefirst bonding agent hardens.
 4. The method according to claim 1 whereinstep A is performed by applying tension to two points on the tensionalmember, the two points disposed between the middle portion andrespective ones of the end portions.
 5. The method according to claim 4further including the step of mechanically holding the tensional stripin a prestressed state following step A and during step B.
 6. The methodaccording to claim 4 further including, prior to step A, the step ofadhering the tensional member to a force introduction body which slidesrelative to the concrete surface during step A.
 7. The method accordingto claim 1 wherein step A comprises engaging the middle portion againstthe concrete surface, with the first bonding agent disposedtherebetween, then prestressing the tensional member by applying tensionto opposite ends of the tensional member, then releasing the tensionforces after the first bonding agent hardens, and then performing stepC.
 8. The method according to claim 1 further comprising the step ofapplying an additional strip-shaped tensional member to the tensionalmember that is bonded to the concrete surface in steps B and C.
 9. Areinforced/prestressed concrete structure in combination with astrip-shaped tensional member joined thereto to strengthen/restore theconcrete structure; the tensional member including a middle portion, andtwo end portions disposed on opposite sides of the tensional member; thetensional member being prestressed only in the middle portion thereof;the prestressed middle portion being joined to a concrete surface of theconcrete structure by a first bonding agent, the end portions beingjoined to the concrete surface by a second bonding agent, the secondbonding agent having a lower deformation modulus than the first bondingagent.
 10. The concrete structure according to claim 9 wherein thetensional member comprises steel fibers.
 11. The concrete structureaccording to claim 9 wherein the tensional member comprises syntheticfibers.
 12. The concrete structure according to claim 9 wherein thetensioning member comprises carbon fibers.